Timed metering valves are preferred for use in a variety of installations in which it is desired to automatically stop flow through the valve after a given length of time has lapsed or a given volume of fluid has been delivered. For example, timed metering valves are useful to control showers in institutions where it is desirable to have valves which are easily activated but which will automatically close to prevent excess waste of water.
In most timed metering valves, the length of time the valve remains open after being actuated is determined by the length of time required for a pressure chamber to fill and to move a member to seal off flow through the valve. The length of time required to fill the chamber is usually controlled by an adjustable needle valve interposed in the bypass circuit. The rate of flow through the bypass circuit and as a result the length of time required to fill the pressure chamber are determined by the clearance between the needle valve and its housing. Although timed metering valves employing needle valves as timing control mechanisms are commercially available, they are not completely satisfactory. In some instances, the timing mechanism fails because the relatively small clearance between the needle and its housing becomes clogged by particulate matter suspended in the fluid being controlled or its "limes" up as a result of the formation of deposits from dissolved impurities in the controlled fluid and as a result, flow through the bypass circuit is reduced or erratic.
Other types of timing mechanisms that have been employed in timed metering valves are devices that operate by the distortion of parts manufactured to close tolerances, miniature conventional valves, and valves that operate by compressing permeable or porous media. However, none of the prior art valves are completely satisfactory as they either require manufacturing to close tolerances; they are sensitive to formation of deposits from dissolved impurities; they are sensitive to the presence of suspended particulate matter; they require expensive linkages or they have no inherent ability to maintain constant flow over a range of supply pressure.